This application relates generally to electrical machines and, more specifically, to apparatus and methods for magnetizing and demagnetizing the permanent magnets used in the rotor assemblies of electrical machines, such as permanent magnets found in the rotor assemblies of wind turbine generators.
Wind turbines can be used to produce electrical energy without the necessity of fossil fuels and can be aggregated in onshore and offshore wind farms connected to the electric power transmission network. Generally, a wind turbine is a rotating machine that converts the kinetic energy of the wind into mechanical energy and, when used for power generation, converts the mechanical energy into electrical power. A conventional horizontal-axis wind turbine includes a tower, a nacelle located at the apex of the tower, and a rotor that is supported in the nacelle by means of a shaft. The shaft couples the rotor either directly or indirectly with a rotor assembly of a generator housed inside the nacelle.
Wind turbine generators and their rotor assemblies can have relatively large diameters of up to ten meters. The primary component of the generator weight is the numerous permanent magnets that constitute the magnetic poles of the generator rotor assembly. In particular, the permanent magnets can weigh on the order of two to five metric tones. Installation of the permanent magnets into the rotor assembly of the generator is challenging, particularly as the size of the generator scales upwardly. The permanent magnets are composed of a material that generates a high-Gauss magnetic field when magnetized and, therefore, a material that is capable of generating strong repulsive or attractive forces with surrounding structures.
Conventionally, the magnetic poles used in the rotor assembly of a wind turbine generator are magnetized before it is installed in the nacelle. One conventional option is to magnetize the permanent magnets, install them into the rotor assembly of the generator, and then install the rotor assembly holding the magnetized permanent magnets into the generator. This assembly option requires use of one mechanical fixture to install the magnetized permanent magnets into the rotor assembly and another mechanical fixture to place the rotor assembly holding the magnetized magnets into the generator housing. A variation on this conventional option is to assemble the wind turbine generator sans permanent magnets and then to use a mechanical fixture to load magnetized permanent magnets into slots defined in the rotor assembly. For either of these conventional assembly options, the strong magnetic fields generated by the permanent magnets interact with surrounding structures, such as nearby ferromagnetic structures of the generator and even other permanent magnets previously installed in the rotor assembly. For example, the stator assembly of the generator may be difficult or impossible to remove because of the forces generated by the magnetized permanent magnets.
Another problem arises during maintenance or service of the generator while located in the nacelle. Generally, generator maintenance may be required, for example, as a result of a lightning strike or a short circuit. Once the permanent magnets are magnetized, the rotor assembly and stator assembly are difficult to remove from the generator because the strong magnetic fields. The difficulty is amplified if a gear box is absent and the rotor assembly of the generator is directly driven by the rotation of the rotor. In this instance, the generator cannot be simply decoupled from a gear box and replaced. The permanent magnets are also practically impossible to demagnetize while positioned within the wind turbine generator.
Improved apparatus and methods are needed to facilitate the installation of an electrical machine, such as a wind turbine generator, as well as improved apparatus and methods for servicing the electrical machine.